Among color display devices used for image display on a computer or television, a display device using a plasma display panel (hereinafter referred to as a “PDP”) has recently been drawing attention, as a large, thin, and light color display device.
A plasma display device using a PDP performs additive color mixing of so-called three primary colors (red, green, and blue) to provide full-color display. For this full-color display, a plasma display device has phosphor layers for emitting the respective three primary colors, i.e. red (R), green (G), and blue (B). Phosphor particles constituting these phosphor layers are exited by ultraviolet light generated in discharge cells of the PDP to generate visible light of respective colors.
Known as compounds used for the phosphors of above respective colors are (Y, Gd)BO3:Eu3+ and Y2O3:Eu3+ for red emission, Zn2SiO4:Mn2+ for green emission, and BaMgAl10O17:Eu2+ for blue emission. Each of these phosphors is fabricated by mixing specific materials and firing the mixture at high temperatures of at least 1,000° C. for solid-phase reaction (see “Phosphor Handbook” p. 219 and 225, Ohm-sha, for example). The phosphor particles obtained by this firing are used after they are milled and classified (average diameter of red and green particles: 2 to 5 μm, average diameter of blue particles: 3 to 10 μm).
The phosphor particles are milled and classified for the following reason. In general, when phosphor layers are formed on a PDP, a technique of screen-printing a paste of phosphor particles of each color is used. In application of the paste, the smaller and more uniform diameters of phosphor particles (i.e. a uniform particle size distribution) can easily provide the smoother coated surface.
In other words, when phosphor particles have smaller and more uniform diameters and shapes approximating to a sphere, the coated surface is smoother. The smoother coated surface increases the packing density of the phosphor particles in a phosphor layer and the emission surface area of the particles, thus alleviating unstableness at address drive. As a result, it is theoretically considered that the luminance of the plasma display device can be increased.
However, the smaller diameters of the phosphor particles increase the surface area of the phosphor and defects of the phosphor. For this reason, a large quantity of water, carbonic acid gas, or organic substances including hydrocarbon are prone to adhere to the surface of the phosphor. Especially for a blue phosphor in which divalent Eu ions mainly emit light, such as Ba1−xMgAl10O17:Eux and Ba1−x−ySryMgAl10O17:Eux, each of these crystal structures has a layer structure (see “Display and Imaging”, 1999, vol. 7, pp 225–234, for example). In these layers, there is oxygen (O) vacancy in the vicinity of layers containing Ba atoms (Ba—O layers) for any particle size. This fact poses a problem: when the particle size is smaller, the amount of defects further increases. (see OYO BUTSURI (Applied Physics), vol. 70, No. 3, 2001, pp 310, for example).
For this reason, water existing in air is selectively adsorbed onto the surface of the Ba—O layer of the phosphor. Therefore, because a large quantity of water is released into a panel in a panel manufacturing process, the water reacts with the phosphor and MgO during discharge. This poses problems of luminance degradation and chromaticity shift (color shift or image burn caused by the chromaticity shift), or decrease in drive voltage margin and increase in discharge voltage. A conventionally devised method to address these problems is coating the entire surface of the phosphor with a crystal of Al2O3, in order to recover the defects in the vicinity of the Ba—O layer (see Japanese Patent Unexamined Publication No. 2001-55567, for example).
However, this method poses another problem: coating the entire surface causes absorption of ultraviolet light and thus decreases the emission luminance of the phosphor, and the ultraviolet light decreases the luminance.